Do we really need Dark Matter and Dark Energy?

I’m still here, but it’s an exam week, so I bring you another guest blogger, Ethan Siegel, who knows way more about dark matter than I probably ever will (a theorist even!). Enjoy his guest post!

This is Ethan from Starts With A Bang! over here at Pamela’s blog, Star Stryder. I was pretty pleased that she came across a couple of press releases and actually thought of me… so let’s have at it! First off, what do the press releases say?

Dr HongSheng Zhao, of the University’s School of Physics and Astronomy, has shown that the puzzling dark matter and its counterpart dark energy may be more closely linked than was previously thought.
…
Dr Zhao reports that, “Dark energy has already revealed its presence by masking as dark matter 60 years ago if we accept that dark matter and dark energy are linked phenomena that share a common origin.”

Really? Well, this is a possibility, but this is certainly not necessarily true. We do know that about 4% of the stuff in the universe is normal matter: what we know as protons, neutrons, and electrons. But although you need dark matter to explain things like

the internal velocities in galaxies and galaxy clusters,

the gravitational lensing data, and

the formation of structure in the universe,

dark energy is only needed to explain why the cosmic expansion rate is larger than it would be if the Universe were made exclusively of matter. They could be related, but they aren’t necessarily. Dark matter affects how gravitational collapse happens, but dark energy affects how things move apart on much larger scales.

But the press release goes on, and contends:

However, the Universe might be absent of dark-matter particles at all. The findings of Dr Zhao are also compatible with an interpretation of the dark component as a modification of the law of gravity rather than particles or energy.

Basically, people don’t like the idea that we only understand 4% of the Universe, and that 96% of the energy in the Universe is made up of stuff (dark matter and dark energy) that we don’t understand at all. So what some scientists attempt to do is to modify the laws of gravity that we know, General Relativity, into something that can mimic the effects of dark matter and dark energy without actually requiring them. But we do need dark matter, and here’s why:

See this picture? This is a picture of two merging galaxy clusters, known as the Bullet Cluster. The bright pink in the picture is X-rays, which come from normal matter. But the blue? That’s a measurement of mass, though gravitational lensing. Normal matter and dark matter both have mass. You tell me, from the image above: where is most of the mass?It isn’t where the normal matter is, that’s for sure. In fact, there isn’t even a consistent way to make a theory of gravity that explains this. Let me quote from this paper that Zhao wrote:

Angus et al. [of which Zhao is one of the coauthors] found that the lensing peaks of the Bullet Cluster could be explained by adding neutrinos in a TeVeS-like modified gravity; the phase space density of neutrinos at the lensing peaks requires 2eV mass to in order not to violate exclusion principle for fermions.

Putting the language issues aside, he’s saying that even if you modify gravity in the best way possible for the Bullet Cluster, you still need 2eV mass neutrinos. From this page, we know that’s more massive than neutrinos are allowed to be. So you still need to add an extra, dark, non-interacting mass to this theory of modified gravity. Know what we call that? Dark Matter!

So the answer to my question is, emphatically, we still need dark matter to be consistent with all of the observations we make. Could dark matter be related to dark energy? Sure, but there’s no evidence that it is. People have also speculated that dark energy is related to neutrinos, axions (a dark matter candidate), and inflation. But at this point, all of it, including this press release, is still just speculation.

I’m happy that people are thinking about it, because these are two of the most interesting topics out there: figuring out what makes the Universe form structure and expand as it does. And yet, the observational evidence for dark matter is overwhelming, even if it still has trouble explaining all of the galactic rotation curves. There’s a lot we still have to learn, but throwing dark matter away isn’t going to be the answer.

â€šÃ„ÃºProfessor Jose Senovilla, and his colleagues at the University of the Basque Country in Bilbao, Spain, have proposed a mind-bending alternative. They propose that there is no such thing as dark energy at all, and weâ€šÃ„Ã´re looking at things backwards. Senovilla proposes that we have been fooled into thinking the expansion of the universe is accelerating, when in reality, time itself is slowing down.â€šÃ„Ã¹

I am personally skeptical.

Steve H

Feb 15, 2008

1) The image posted is a computer generated “cartoon” and not something real. It was produced by a computer model which may, or may not, represent physical reality.

2) Lable me as a skeptic on the issue of dark matter and energy. Something still does not “smell” right to me, and I have been involved in astronomy for 35 years now.

3) Horrible!

If only one other student had been armed, the number of deaths would have been minimized.

A “GUN FREE ZONE” is once again responsible for the deaths of multiple people. When are the stupid people going to learn?

1) The image you see is a real visible light image of the Bullet Cluster. The pink is the data from the Chandra X-ray telescope overlayed, and the blue is inferred mass from the gravitational lensing data in the visible light image. There is nothing computer-modeled about it.

2) Being a skeptic is wonderful, but I’m going to call you on labeling yourself a skeptic and your argument against dark matter is in its odor. The reality, like it or not, is that we need there to be more mass in the Universe than we observe, and moreover, more mass than exists in the Universe in normal matter. Dark energy could be a lot of other things, and is really just a name we give to the fact that the Universe expands at a different rate than we expect from the matter in it. But you want to get rid of dark matter? You can’t. No matter how you modify gravity, there’s no theory that can account for all the structure we observe. There isn’t. People are trying. Dr. Zhao is trying. He published a bunch of papers on his attempts and then even says, as quoted above, he still needs dark matter to explain it.

3) What happened at NIU is, indeed, horrible. I’m sure that everywhere people’s hearts and minds are going out to the people whose lives have been affected by this tragedy.

Brian Hunt:

This is a very difficult question. Dark energy, remember, isn’t a fluid, isn’t a substance, isn’t a particle. It’s a name we give to a phenomenon. It’s consistent with being a new type of energy, but the truth of the matter is we really don’t know much about it at all.

Your article makes a number of suppositions, namely:

We live in a Universe with more than 4 dimensions (three space and one time).
In that Universe, it’s possible to have more or fewer than one time dimension.
Time dimensions can turn into space dimensions, and vice versa.

Now, objectively, there is no experimental or observational evidence for any of these. You can show that these things are mathematically possible, but so are octonions (there’s a word for google), and we know that they have no physical analogue.

But is it possible? Sure; there are no constraints on it at present. I imagine that this would mimic many of the effects of variable-speed-of-light scenarios, but those do have constraints on them. In any case, I’m not losing any sleep over that possibility, but I don’t know enough to say “that’s impossible!”

-Ethan

Brian Hunt

Feb 16, 2008

Huge thanks for addressing my question Ethan. I’ve been trying to get an opinion on that article from someone with a PhD since its debut.

Steve H

Feb 18, 2008

Ethan Siegel;

1) You stated in your first comment that there was nothing computer-modeled about the image. At the same time, you stated that “the blue was inferred mass from the gravitational lensing data in the visible light image.”

As I said, this was not an actual image of dark matter, but the result of a computer model. It is important that people understand the difference.

Computer models can only reflect current understanding, and as new knowledge is learned, the computer models will be altered.

2) I do not call myself a skeptic, because I accept and understand the difficulties in explaining such simple things as the radial velocity of galaxies.

These are physical measurements that can not be denied.

As you measure the velocity of stars within a galaxy, from the center to the outer edges, the radial velocities do not follow the inverse squared law. If anything, galaxies behave more like coffee being stirred.

To explain the fluid like behaviour of galaxies, the concept of dark matter was created.

At the moment, dark matter is only an attempt to explain the fluid behaviour of what we observe.

Again, reconstructing where the mass is isn’t a result of a computer model. It’s a result of taking an image and saying “based on our theory of gravity, how much mass does there need to be, and where, to give us the observed results.” Just because it isn’t a direct image of the dark matter doesn’t mean it isn’t reliable. It’s as reliable as the theory of gravity that we have, and general relativity hasn’t been wrong, once, yet. If you’re saying that it’s an analysis of the data, that’s true. But it isn’t a model or a simulation; it could be done by hand just as easily.

And I will reiterate that dark matter is not an attempt to explain the fluid behavior of what we observe, although it can explain the rotation curves of galaxies, for one. It can also explain cluster velocity dispersions, the suppression of silk damping in the power spectrum, the shape and location of the acoustic peaks in the cosmic microwave background, the strong and weak gravitational lensing data from galaxies and clusters, the Hubble expansion rate, the formation of large scale structure in the Universe, and it’s all consistent with everything else we do and do not observe. Dark matter is one extra parameter, and it explains all of these things without messing up big bang nucleosynthesis, galaxy formation, stellar evolution, our theories of gravity, or anything else that we can think to measure. It’s not that dark matter is an attempt: it’s that dark matter is the only thing that works everywhere.

It’s tough, isn’t it, when a lot of smart people are telling you compelling, but competing things. It’s very difficult when you have one standard set of laws, that we know work, but they don’t explain everything.

What do you do? Well, if the observations are robust, you need to explain it, which means you need either a new law or new set of laws, so you see what they predict, and how those new predictions line up with other things. That’s why dark matter works so well, for instance. But as for dark energy… yeesh. There’s too much leeway right now to really narrow things down. So people can write about even very outlandish possibilities, and they’re still within the realm of possibility. As we learn more, we learn what can work and what can’t. For dark matter, we know quite a lot. For dark energy… ask me again in a year or two and see if I give you the same answer. 😉